1. Field of the Invention
This invention concerns a method of operating an air-supplied type coating booth in which conditioned air supplied from an air supply blower to a plenum chamber is enforced through a filter at a predetermined flow velocity downwardly to the inside of a tunnel-shaped coating booth and the air in the coating booth is drawn to discharge together with paint mists, evaporated organic solvents and the likes to the beneath of a booth floor by way of an exhaust blower.
2. Description of the Prior Art
In a relatively large-scaled air-supplied type coating booth used, e.g., for coating car bodies conveyed successively on a conveyor, conditioned air supplied from an air supply blower to a plenum chamber is enforced to the inside of a tunnel-shaped coating booth disposed between a pretreatment device for coating and a drying furnace for coated articles. The air thus enforced downwardly to the coating booth is drawn to discharge to the beneath of the booth floor together with paint mists, evaporated organic solvents and the likes by means of an exhaust blower, so that the paint mists, dusts or the likes which would otherwise give undesired effects on the coated films may be prevented from scattering and drifting upwardly, thereby maintaining favorable coating quality, as well as keeping the health of operators who make preparatory work for the coating or conduct manual spray coating within the coating booth.
By the way, in the coating booth of the type as described above, if the flow rate of air supplied from the air supply blower and that of exhaust air discharged by the exhaust blower are different, external air containing dusts or the likes would intrude to the inside of the coating booth through the inlet and the exit opened at both ends thereof to degrade the quality of coated films applied on the car bodies, or air contaminated with paint mists, evaporated organic solvents and the likes in the coating booth would flow out of the coating booth through the inlet and the exit to contaminate the circumstantial peripheries such as a pre-treatment device for coating or a drying furnace for coated articles. Accordingly, the air supply blower and the exhaust blower have usually been driven at a predetermined constant number of rotation or constant angle of blade, so that the flow rate of the conditioned air supplied from the supply blower and the flow rate of the exhaust air drawn to discharge by the exhaust blower to the beneath of the floor of the coating booth are balanced with each other.
However, since a plurality of slits are formed as air sucking ports to the floor of the air-supplied type coating booth adapted to draw the air in the coating booth to the beneath of the floor and car bodies are successively conveyed passing over the slits, the sucking ports are closed with the car bodies. Accordingly, the flow rate of the exhaust air is decreased, in other words, flow rate of the air supplies is increased relatively thereby causing the contaminated air in the coating booth to flow outwardly through the inlet and the exit depending on the case.
Thus, in the case where the number of rotation or the angle of blade of the air supply blower and the exhaust blower is previously set constant as in the prior art, the balance between the flow rates of the supplied air and the exhaust air is lost in accordance with the increase or decrease in the quantity of coated articles such as car bodies conveyed through the coating booth, and air may possibly flow into and out of the coating booth through the inlet and exit thereof.
In view of the above, the present inventors et al. have already proposed a method of operating an air-supplied type coating booth, which comprises detecting the flow velocity of air flowing into or out of the coating booth through the inlet and the exit thereof, variably controlling the flow rate of the exhaust air discharged from an exhaust blower in accordance with the flow rate of air stream flowing into and out of the booth calculated based on the flow velocity thereby keeping a balance between the flow rate of the exhaust air discharged from the exhaust blower and the flow rate of the air supplied from the supply blower thus preventing the air stream from flowing into and out of the coating booth through the inlet and the exit thereof (U.S. patent application Ser. No. 789,769 filed on Oct. 21, 1985 and now allowed, entitle as "Method of operating an air feed type spray booth").
In the operation method of our proposed prior patent application, if the balance between the flow rate of the supplied air and the flow rate of the exhaust air is lost due to the increase or decrease in the number of coated articles to be conveyed and air stream tend to flow into and out of the coating booth through the inlet and the exit thereof, the flow rate of the exhaust blower is controlled to increase or decrease depending on the detected flow velocity to prevent the air stream from flowing into and out of the booth through the inlet and the exit.
However, it has been found by our subsequent studies and experiments that fluctuations, disturbances or, depending on the case, even swirlings are resulted in the air streams at the inlet and the exit of the coating booth under the effect of external atmosphere or the coated objects to be conveyed into and out of the booth and they cause irregular variations in the detected flow velocity.
That is, it has been found that when the flow velocity at the inlet and the exit is continously measured by flow velocity sensors, measured signals inevitably contain primary variations fluctuating at relatively shorter periods, for example, about from 1 to 4 seconds caused, for example, by disturbances in the air streams upon contact with the flow velocity sensors or the characteristics of the sensors per se and secondary variations fluctuating at relatively longer periods caused, for example, by disturbances in the air streams when the coated objects are conveyed into and out of the booth. The measured signals are also varied abruptly when a sudden wind flows into and out of the coating booth briefly through the inlet and the exit thereof.
In addition, when the flow velocity of the air streams flowing out of the booth is detected, for example, as a positive signal while the flow velocity of the air streams flowing into the booth is detected as a negative signal, the foregoing disturbances in the air streams or the likes may some time cause not only the variation in the absolute value but also the change for the polarity (direction) of the air streams, by which a detection signal varying, for example, on the positive level may be changed suddenly into the negative level.
In this case, if the flow rate of the exhaust air discharged from the exhaust blower is intended to control accurately following after every minute and frequent variations, fine and frequent adjustment is required for controlling the operation of the exhaust blower. However, it is troublesome and difficult for the blower which usually has a considerable inertia, and the desirable balance between the flow rates of air supply and exhaust may rather be lost depending on the case. Thus, the air stream flowing into or out of the coating booth through the inlet and the exit thereof may some time be promoted rather than hindered.
For preventing the air flow from flowing into or out of the coating booth through the inlet and the exit thereof by the control for the operation of the exhaust blower, there occurs another problem.
That is, if the exhaust blower is controlled for the minute or fine disturbances resulted at the inlet and the exit of the booth that are caused only instantaneously by external disturbances having no substantial concerns with the balance between the flow rate of the supply blower and that of the exhaust blower, over-control will often be resulted to the exhaust blower making the operation instable and rather causing unfavorable effects on the balance in the coating booth.
Thus, although the exhaust blower is usually controlled for suppressing the air streams to flow out or into the coating booth through the inlet and the exit, it is desirable that the operation of the blower be kept steadly as much as possible unless it is required, so as not to disturb the balanced state once attained between the flow rate of the air supply blower and the exhaust blower.